GB2159672A

GB2159672A - Electronically commutated reluctance motors

Info

Publication number: GB2159672A
Application number: GB08507065A
Authority: GB
Inventors: Timothy John Eastham Miller; Allan Barr Plunkett; Robert Louis Steigerwald
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1984-05-31
Filing date: 1985-03-19
Publication date: 1985-12-04
Also published as: US4684867A; IT8520481A0; IT1184778B; DE3518926A1; FR2565435B1; JPS60257786A; FR2565435A1; GB8507065D0

Description

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SPECIFICATION Switched reluctance motors

5 Switched reluctance motors conventionally have poles or teeth on both the stator and the rotor (i.e. they are doubly salient). There are phase windings on the stator but no windings on the rotor. Each pair of diammetrically op-10 posite stator poles is connected in series to form one phase of the switched reluctance motor.

Torque is produced by switching current on in each phase winding in a predetermined 15 sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each 20 phase at the commutation point before the rotor poles nearest the stator poles of that phase rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque. Quick 25 reduction of the phase current to zero helps avoid negative torque.

The torque developed is independent of the direction of current flow. Unidirectional current pulses synchronized with rotor movement 30 can be generated in a converter using a single unidirectional current switching element such as a thyristor or transistor in each phase.

Each time a phase of the switched reluctance motor is switched on by closing a 35 switch in a converter, current flows in the stator winding, providing energy from a dc supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate towards 40 a minimum reluctance configuration and partly into a magnetic field. When the switch is opened, part of the stored magnetic energy is converted to mechanical output and the remainder of the energy is preferably returned 45 to the dc source.

In prior art switched reluctance motors, efficiency is reduced because of unrecovered energy in the magnetic field. It is known to recover some of this energy by using a bifilar 50 winding to allow current to return to the dc source after the main switching device turns off. The bifilar winding permits this energy recovery without having recourse to alternative circuits that require two switching devices 55 and two freewheel diodes in each phase. However, bifilar windings have the disadvantages of high cost, poor winding space utilization and a doubling in the number of terminal connections.

60 It is also known to use a single winding with a bipolar power supply. A bipolar supply is undesirable because the available dc voltage cannot be utilized efficiently and because the base drive and other control circuits have 65 to be supplied through isolation transformers,

increasing overall costs.

It is an object of the present invention to provide an improved method for commutating a switched reluctance drive which is not subject to the foregoing disadvantages.

According to the invention there is provided a commutation method for a switched reluctance drive and converter circuit, wherein the decay rate of current is increased by transferring freewheeling current to a charge storage device having a voltage greater than the dc source voltage. The charge may then be recovered by actively bleeding at least a portion of the charge back to the dc source.

A recovery circuit is provided comprising a charge storage device coupled to each phase through a phase unidirectional device. The recovery circuit may also include means for returning charge to the dc source to provide regenerative recovery of the residual magnetic stored energy contained in a phase winding of the switched reluctance drive at the end of a working stroke.

Maximum energy recovery from the switched reluctance drive can be achieved employing a single switching device and a single freewheel device per phase, with a non-bifilar winding in each phase and a unipolar supply.

The phase current of the switched reluctance drive is thereby quickly reduced to zero after the commutation point in order to eliminate negative torque.

An improved converter circuit for recovering magnetic stored energy is also provided particularly a converter circuit for a switched reluctance drive which increases the decay rate of current in a phase following the commutation point.

In the accompanying drawings, by way of example only:-

Figure 1 is an illustration of a conventional switched reluctance drive;

Figure 2 shows a typical flux-linkage/current trajectory for a phase winding of a conventional switched reluctance drive;

Figures 3A and 3B show exemplary prior art converter circuits using only one main switching device per phase;

Figure 4 is a graphic illustration comparing the phase current waveforms of a circuit embodying the present invention and the prior art;

Figure 5 is a converter circuit embodying the present invention;

Figure 6 illustrates the dump capacitor voltage waveform of the converter circuit of Fig.

5.

Figure 7 is another circuit embodying the present invention;

Figure 8 is yet another converter circuit embodying the present invention

Figure 9 is a block diagram of a system used to control a converter circuit embodying the invention; and

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Figure TO shows the switching angle calculator of Fig. 9 in greater detail.

Fig. 1 shows a portion of a switched reluctance motor in cross-section and a converter 5 circuit for one phase of the motor. A laminated iron stator 100 has a plurality of stator pole pairs, including a pair comprised of stator poles 110 and 111. A primary coil 11 and a secondary coil 12 are each wound in series on 1 o stator poles 110 and 111. The coils are energized by a converter 65 as known in the art and described with reference to Fig. 3A.

A laminated iron rotor 101 has a plurality of pairs of rotor poles including a pair com-15 prised of rotor poles 113 and 114. Rotor 101 is fastened to a shaft 105 and drives a shaft position sensor 61.

Although the following discussion references only one motor phase, it will be clear 20 that all phases behave identically except that their energization by the converter may be phased by equal increments of rotor rotation.

The converter circuit of Fig. 3A for one phase of the motor shown in Fig. 1 comprises 25 a bifilar winding 10 having primary coil 11 and secondary coil 12 both wound on stator poles 110 and 111 as shown in Fig. 1, wherein an increasing with time positive current entering a dotted terminal of one coil 30 produces an open-circuit voltage across the second coil which is positive at the dotted terminal of the second coil. Secondary coil 12 is conencted in series with a diode 22 and a dc source 15, the dc source and the diode 35 being connected in series opposition. The collector of a bipolar transistor 20 is connected to the dotted terminal of coil 11 and the emitter of transistor 20 is connected to the anode of diode 22, while the non-dotted ter-40 minal of coil 11 is connected to the dc source 1 5 and the dotted terminal of coil 1 2. The base of transistor 20 is connected to a control circuit, not shown.

The trajectory of the flux-linkage versus 45 current curve for the converter circuit shown in Fig. 3A is illustrated in Fig. 2. Curves OX and OY represent the magnetization characteristics of one phase of the motor shown in Fig. 1 when the rotor is stationary in the minimum 50 and maximum inductance positions, respectively. Curve OA is the trajectory in a running motor resulting from the flow of current in primary coil 11 of bifilar winding 10 when switching element 20 is conducting. While 55 switching element 20 conducts, unidirectional conduction device 22 prevents current from flowing in secondary coil 12. At commutation point A, current is flowing into primary 11 from dc source 1 5. At point A, transistor 20 60 is switched off through circuitry (not shown) connected to the base electrode, in a manner well-known in the art and described below with reference to Fig. 9, interrupting current flow in primary 11. The resulting voltage 65 induced in secondary 12 produces a current flow out of secondary 12 at the dot, which is returned to dc source 15 through forward-connected diode 22. As current and flux-ttnkage decrease, the flux-linkage versus current curve returns to point 0.

The converter circuit of Fig. 3B comprises two phase windings 13 and 14 and a bipolar dc source 16. The collector of a bipolar transistor 23 is connected to the positive terminal of a first supply portion 17 of bipolar source 16. Phase winding 13 is connected between the emitter of transistor 23 and a junction of the negative terminal of first supply portion 17 and the positive terminal of a second supply portion 18. The emitter of a bipolar transistor 24 is connected to the negative terminal of second supply portion 18. Phase winding 14 is connected between the collector of transistor 24 and the positive terminal of second supply portion 18. A diode 26 connects the collector of transistor 24 to the positive terminal of first supply portion 17. A diode 25 connects the emitter of transistor 23 to the negative terminal of second supply portion 18. Both diodes 25 and 26 are connected in opposition to dc source 16.

Each phase 1 3 and 14 achieves the same curve trajectory as shown in Fig. 2. The circuit of Fig. 3B differs from the circuit of Fig. 3A in that when a transistor is switched off current still flows in the respective winding, but now it flows through freewheeling diode 24 or 25 into a portion of bipolar dc source 16 other than the one which initially supplied the current.

At commutation point A, using either of the converters in Fig. 3, there is stored in the phase winding magnetic energy equal to the sum of areas F and W, illustrated by cross-hatching in Fig. 2. The size of area W is related to the mechanical output energy of the working stroke, i.e. one traversal of the trajectory. Area F represents the energy which may be recovered and returned to the dc supply. Although the converter circuits shown in Figs. 3A and 3B recover most of this energy, they suffer from the disadvantages previously discussed.

A current waveform for the prior art converter circuits of Fig. 3 is shown in Fig. 4. Curve 30 represents the increase of current to commutation point A. Current decay in the prior art converter circuits is shown by curve 31. If this current has not reduced to zero by the time that the rotor poles are aligned with the stator poles, then the rate of change of inductance becomes negative causing an electromotive force which sustains or even increases the phase current. This increase in current, shown as a hump 32 in curve 31, generates even more negative torque.

The improved current decay waveform 33 in Fig. 4 may be achieved by the method of the present invention which avoids generation of negative torque while efficiently recovering

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residual magnetic energy. The decay rate of phase current after the commutation point is increased so as to quickly bring the phase current to zero by the step of allowing the 5 current to freewheel into a charge storage device which has a voltage higher than the voltage of the dc source either at the time that freewheeling begins or during freewheeling as the freewheeling current increases the voltage 10 in the charge storage device. Recovery of the residual magnetic energy is realized by the further step of selectively bleeding charge stored in the charge storage device back to the dc source.

15 One embodiment of the converter circuit of the present invention, and which generates waveform 33 of Fig. 4, is shown schematically in Fig. 5. Unipolar dc source 1 5 may be comprised of a full-wave rectified ac source as 20 shown. A dc link capacitor 35 is connected across source 1 5. Phase windings 40, 42 and 44, respectively, are each connected in series with a transistor 41, 43 and 45, respectively, across dc source 15. The bases of transistors 25 41, 43 and 45 are connected to a firing pulse generator 64 of the control circuit described more fully below. The emitters of transistors 41, 43 and 45 are connected to the negative terminal of source 15. Transistors 41, 43 and 30 45 constitute the main switching devices of the converter circuit and may comprise bipolar transistors, FETs, SCRs, insulated gate transistors or gate turn-off thyristors.

The converter circuit further comprises an 35 energy recovery circuit. The charge storage device of the invention is shown, in Fig. 5, as a dump capacitor 36 having a first terminal connected to the negative terminal of source 1 5. Thyristors 46, 47 and 48, respectively, 40 connect phase windings 40, 42 and 44, respectively, to a second terminal of dump capacitor 36. Each thyristor 46, 47 and 48 has its anode connected to its respective phase winding, its cathode connected to 45 dump capacitor 36, and its gate connected to pulse generator 64. The second terminal of dump capacitor 36 is coupled through an inductor 50 to the anode of a thyristor 49 which has its cathode connected to the posi-50 tive terminal of source 15 and its gate connected to pulse generator 64.

Since the operation of all phases is identical, the first phase comprised of phase winding 40 and transistor 41 will be considered as 55 a representative example. When transistor 41 switches on, the phase current builds up in the normal way as previously described regarding Figs. 2 and 3A. When transistor 41 turns off at the commutation point, its associ-60 ated phase current is allowed to freewheel through thyristor 46 into dump capacitor 36. Thyristor 46 is gated on at the same instant that transistor 41 is turned off or slightly before. All thyristors and transistors are con-65 trolled by firing pulse generator 64, described below with reference to Figs. 9 and 10.

In the embodiment of Fig. 5, the capacitive value of dump capacitor 36 is chosen such that the free-wheeling current charges dump capacitor 36 to a voltage level of about 2 to 3 times the dc source voltage, Vs. The capacitive value of link capacitor 35 is 5 to 10 times that value. The resulting voltage waveform of dump capacitor 36 is shown in Fig. 6. At commutation point A, dump capacitor 36 is charged to some voltage less than Vs. As the phase current freewheels, the dump capacitor voltage rises. When phase current drops to zero at point Z, the dump capacitor voltage is at a maximum, i.e. 2 to 3 times Vs, and thyristor 46 is gated off.

Also at point Z, thyristor 49 is gated on to restore the increased charge on dump capacitor 36 to link capacitor 35. The charge is transferred in a resonant manper in the series resonant loop comprising dunHp capacitor 36, inductor 50 and link capacitor 35. Thus, the dump capacitor voltage falls to a level well below Vs, as shown in Fig. 6. Thyristor 49 is gated off when the resonant current drops to zero. Thyristors 46, 47 and 48 prevent the respective phase windings from conducting as soon as the dump\capacitor voltage drops below Vs. At this point, dump capacitor 36 is ready to receive freewheeling current from another phase.

The embodiment of a converter circuit shown in Fig. 7 is a modification of the circuit in Fig. 5. Diodes 51, 52 and 53 connect phase windings 40, 42 and 44, respectively, with dump capacitor 36. A FET 55 has its drain connected to dump capacitor 36, its source connected to inductor 50 and its gate connected to a control comprised of a comparator 57 having a resistor 58 connected to the one input terminal and a potentiometer 59 connected to another input terminal. A diode 54 is also included with its cathode connected to inductor 50 and its anode connected to the negative terminal of dc source 1 5, as shown.

When the circuit of Fig. 7 is first energized by dc source 1 5, current flows through each phase winding and its respective diode into dump capacitor 36. The voltage across dump capacitor 36 rises to substantially 2VS due to resonance. Dump capacitor 36 is charged in just a few milliseconds. Diodes 51, 52 and 53 become reverse biased when current flow in phase windings 40, 42 and 44 ceases. Diodes may be used instead of thyristors because the dump capacitor voltage is always greater than Vs.

Further operation of the converter of Fig. 7 is similar to the previously described circuit. Freewheeling current charges dump capacitor 36 in the energy recovery circuit. However, instead of having a periodic resonant discharge of dump capacitor 36, a chopper circuit comprising FET 55, inductor 50 and diode 54 is used to bleed excess energy from

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dump capacitor 36 back to link capacitor 35, maintaining a roughly constant voltage on dump capacitor 36.

When the dump capacitor voltage exceeds a 5 predetermined level (on the order of twice Vs and adjustable through potentiometer 59), FET 55 is turned on by comparator 57 and current flows from dump capacitor 36 through inductor 50 and into link capacitor 10 35. As the current reaches a maximum, FET

55 turns off, preventing the voltage across dump capacitor 36 from falling below Vs and allowing it to fall only slightly below the predetermined level. After FET 55 turns off,

15 inductor 50 continues to conduct through diode 54 until all of the energy is returned to link capacitor 35, which then makes the energy available to be transformed into useful work. In the meantime, other phases may be 20 free-wheeling, supplying more energy to dump capacitor 36.

The advantage in keeping the dump capacitor voltage, VCKjump greater than Vs lies in the resulting ability to reduce phase current to 25 zero in a shorter time. During freewheeling of phase current into dump capacitor 36

d*/dt = V-Vc.dump,

30 where SF represents phase flux-linkage. According to this relationship, the higher the dump capacitor voltage, the higher the rate of decrease of phase flux-linkage. Thus, the converter circuit as described also achieves rapid 35 extinction of the phase current.

A further embodiment of the converter circuit, as shown in Fig. 8, is obtained by replacing inductor 50 in Fig. 5 with a resistor

56 and replacing the thyristors 46, 47 and 40 48, respectively, in each phase with diodes

51, 52 and 53, respectively. In this case, discharge of dump capacitor 36 through resistor 56 into link capacitor 35 is not resonant. Therefore, Vcdump remains greater than or 45 equal to Vs, allowing diodes to be used in each of the phases. Efficiency is lowered due to power loss in resistor 56, but the cost of the converter is lowered and its control is simplified.

50 Control of the main switching devices in each phase will be described with reference to Figs. 9 and 10. It will be apparent to those skilled in the art that the gating signals for the switching transistors or thyristors in the vari-55 ous converter circuit embodiments may be easily derived from the switching signals described below.

Fig. 9 is a block diagram of apparatus which senses rotor position, calculates switch-60 ing angles and generates phase firing signals. The switching angles are calculated at which (1) the main switching device or devices turn on to energized phase windings; and (2) the main device or devices turn off allowing the 65 motor current to freewheel.

The shaft position sensor 61 may be a device mechanically coupled to shaft 105 and which provides a fixed number of electrical pulses with each rotor revolution. The position of the rotor is determined by counting the pulses. An index pulse may be provided so that the absolute position can be ascertained.

Interface circuit 62 is a buffer amplifier which conditions and "squares up" (i.e., reshapes into rectangular form) the pulses from position sensor 61. Typically, circuit 62 may be a Schmitt trigger circuit.

Switching angle calculator (SAC) 63, shown in detail in Fig. 10, is comprised of counters that divide down the sensor pulses received from interface 62. For example, if position sensor 61 of Fig. 9 produces 2400 pulses per revolution and the rotor has 6 poles, then a first counter 70 in SAC 63 divides by 400 to obtain 6 rotor position pulses per revolution.

Each of these 6 pulses represents a reference pulse associated with a respective rotor pole and is used in resetting a respective one of a plurality of second counters 71a, 71b and 71c each associated with a separate phase, respectively. Each second counter counts down an interval corresponding to the angle through which the rotor is required to move before the respective phase current is switched on. This angle is stored in a memory : (not shown), typically a microprocessor based memory. The reset signals for the respective phases coming from the rotor position pulses are delayed by appropriate amounts in delays 74 and 75 corresponding to the angular displacements between the phases, with a single (nonbifilar) motor phase winding in each phase.

Each respective one of a plurality of third counters 72a-72c, each associated with a separate phase, respectively, is reset by the pulses resetting second counters 71a, 71b and 71c, respectively. The third counters count down the intervals corresponding to the angle at which each phase current is switched off. A logic circuit 76 uses pulses from counters 71-72 to control a firing pulse generator 64, shown in Fig. 9. Logic circuit 76 may be an AND gate for each phase with the input from third counters 72a-72c inverted, as shown for one phase in Fig. 10. The process is repeated in each phase at the next occurrence of a rotor position pulse which resets the counters.

In a microprocessor based control scheme the counts that are loaded into the various counters can be stored in memories or registers and their values can be changed at frequent intervals. The microprocessor may be programmed with a strategy that takes into account speed, load and command signals.

Firing pulse generator 64 is a standard type of circuit available as a single field-effect transistor in each phase, which amplifies the logic level signals received from SAC 63 to provide

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sufficient power to the converter switching devices to turn them on and off, for driving motor 60. In drives of low power (typically less than 100 W) the power amplification 5 provided by firing pulse generator 64 is not necessary and the converter switching devices can be turned on and off directly by logic level signals from SAC 63.

The present invention provides a method of 10 commutation with appropriate converter circuits for a switched reluctance drive. Regenerative recovery of residual magnetic energy and rapid decay of phase current are achieved by causing freewheeling current to charge a 15 dump capacitor and then selectively bleeding the stored charge back to the dc source. This is accomplished with a unipolar dc source, nonbifilar windings and only one main switching device per phase.

20 While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, 25 changes, departures, substitutions and partial and full equivalents will now occur to those skilled in the art without departing from the invention. Accordingly, it is intended that the invention be limited only by the scope of the 30 appended claims.

Claims

1. A method for increasing the decay rate of current in a phase winding of a switched

35 reluctance drive supplied by a dc source during freewheeling after the commutation point, said method comprising the step of:

transferring the current from said phase winding during freewheeling to a charge sto-40 rage device having a voltage greater than the dc source voltage.

2. A method for recovering energy released from a phase winding of a switched reluctance drive during free-wheeling of said

45 phase winding after the commutation point, said phase winding being connected to a dc source, said method comprising the steps of:

(1) transferring the charge released from said phase winding during freewheeling to a

50 charge storage device; and

(2) selectively bleeding at least a portion of the charge in said storage device back to said dc source.

3. The method of claim 2 wherein the 55 voltage across said storage device is at all times greater than the voltage across said dc source.

4. The method of claim 2 wherein the voltage across said storage device is greater

60 than the voltage across said dc source when said bleeding is initiated.

5. A method for recovering energy released from the respective phase windings of a switched reluctance drive during freewheel-

65 ing of each of said phase windings after the commutation point thereof, said phases being connected to each other in parallel and to a dc source, said method comprising the steps of:

(1) transferring the charge released from each of said phase windings during its respective freewheeling interval to a charge storage device; and

6. A recovery circuit in a converter for a phase winding of a switched reluctance drive, said winding having a first side connected to the positive terminal of a dc source, said recovery circuit comprising:

a phase unidirectional conducting device having its anode connected to a second side of said phase winding; and a charge storage device having a first side connected to the cathode of said phase unidirectional conducting device and a second side connected to the negative terminal of said source.

7. A recovery circuit in accordance with claim 6 wherein said phase unidirectional conducting device comprises a thyristor which is turned on only during freewheeling of said phase winding, said recovery circuit further including the series combination of a selectively operable recovery thyristor and an inductor connected between said first side of said charge storage device and said positive terminal of said source.

8. A recovery circuit in accordance with claim 6 further including the series combination of a selectively operable recovery thyristor and a resistor connected between said first side of said charge storage device and said positive terminal of said source.

9. A recovery circuit in accordance with claim 8 wherein said recovery thyristor is operated when the voltage across said charge storage device exceeds a predetermined level.

10. A recovery circuit in a converter for a phase winding of a switched reluctance drive, said winding having a first side connected to the positive terminal of a dc source, said recovery circuit comprising:

a phase unidirectional conducting device having its anode connected to a second side of said phase winding;

a charge storage device having a first side connected to the cathode of said phase unidirectional conducting device and a second side connected to the negative terminal of said dc source;

a chopper switching element having a first side connected to said first side of said charge storage device;

a chopper unidirectional conducting element having its cathode connected to a second side of said chopper switching element and having its anode connected to said negative terminal; and

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an inductor connected between said second side of said chopper switching element and the positive terminal of said source.

11. A recovery circuit in accordance with 5 claim 10 wherein said dc source further includes a capacitor connected thereacross.

12. A recovery circuit in accordance with claim 10 wherein said chopper switching element is controlled by a chopper control coup-

10 led thereto whereby said chopper switching element conducts charge from said charge storage device through said inductor to the positive terminal of said source when the voltage across said charge storage device ex-15 ceeds a predetermined level and becomes non-conductive before the voltage across said charge storage device falls below the level of the voltage across said dc source.

13. A recovery circuit in accordance with 20 claim 11 wherein said chopper switching element is controlled by a chopper control coupled thereto whereby said chopper switching element conducts charge from said charge storage device through said inductor to said

25 capacitor when the voltage across said charge storage device exceeds a predetermined level and becomes nonconductive before the voltage across said charge storage device falls below the level of the voltage across said dc 30 source.

14. A recovery circuit in accordance with claim 12 wherein said chopper switching element comprises an FET with its drain and source being said first and second sides of

35 said switching element, respectively, and wherein said chopper control comprises a comparator with one input coupled to said charge storage device and another input coupled to the output of a voltage divider con-40 nected across said dc source, the output of said comparator being coupled to the gate of said FET.

15. A recovery circuit in accordance with claim 10 wherein said phase unidirectional

45 conducting device comprises a diode.

16. A recovery circuit in accordance with claim 10 further including additional phase unidirectional conducting devices, each connecting an additional phase winding to said

50 charge storage device.

17. A switched reluctance drive comprising:

a plurality of pairs of stator poles, each pole of each pair being wound with a single wind-55 ing, the single windings of each pair connected in series to form a phase winding;

a rotor having rotor poles;

a unipolar dc source; and a converter for energizing each of said 60 phase windings, said converter including a plurality of main switching devices, each one of said main switching devices, respectively, connected in series with one of said phase windings, respectively, an energy recovery cir-65 cuit having a dump capacitor, said energy recovery circuit connected across said dc source, and a plurality of freewheeling unidirectional conduction device, each one of said unidirectional conducting devices, respec-70 tively, connecting one of said phase windings, respectively, to said dump capacitor;

whereby residual magnetic energy is efficiently restored to said unipolar dc source.

Printed in the United Kingdom for

Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.